A need exists to develop aircraft engine propulsion nozzles with reduced acoustic noise signatures for low speed operations around airports and the surrounding community. In 2007 more stringent FAR36 Stage 4 regulations for community noise standard were enabled which further restrict allowable noise emissions from aircraft. Currently more stringent “Stage 5” noise regulations are anticipated for new transport category aircraft making application for certification after 2017. Supersonic aircraft have more extreme challenges in meeting anticipated noise limitations than subsonic aircraft.
The major source of noise from aircraft is the high speed jet emanating from the exhaust nozzles of turbofan engines. Experimental work dating to the 1970's established the potential for high radius plug nozzles to reduce jet noise with relatively modest reductions in thrust efficiency, See “Initial Results of a Porous Plug Nozzle for Supersonic Jet Noise Suppression”, by Maestrello, NASA TM-78802, 1978, and “Jet Noise Suppression by Porous Plug Nozzles” by Bauer, Kibens, and Wlezian, NASA Contractor report 3613, 1982.
One of the phenomena leading to reduced noise relative to a simple circular nozzle is the greater shear area relative to the thickness of the jet, leading to shorter mixing length between the high speed jet and the lower speed external flow passing around the outside of the nozzle and joining the jet at the exit plane. Much of the acoustic work on plug nozzles has focused on “porous” plugs which feature a hollow plug drilled with arrays of small holes, to suppress “screech” tones associated with shock cells of high pressure ratio supersonic aircraft nozzles. Test results also showed significant noise reductions with solid plugs at the lower pressure ratios associated with subsonic aircraft. The embodiments described herein use a different approach, though this different approach can be used in combination with such porous plugs.
A critical performance parameter for nozzles is the nozzle pressure ratio, which is the total pressure of the exhaust flow emanating from the engine divided by the ambient pressure surrounding the aircraft. Nozzle pressure ratios greater than unity will result in a net flow out through the exhaust nozzle. In order to create net thrust on the aircraft, the velocity exiting the nozzle system must be greater than the velocity entering the engine inlet, thus with increasing aircraft velocities the exhaust exit velocity must increase to create thrust, and requires higher nozzle pressure ratios with increased speed.
FIG. 1 is a cross-section view of a simple convergent nozzle common on subsonic aircraft consisting of an outer nacelle enclosing a turbofan or turbojet engine 2, air inlet 3, and exit duct 4. Exhaust flow 21 flows aft through the duct to exit through the nozzle exit 18. The minimum cross-section area 18 occurs near the exit plane of the nozzle 17.
A nozzle pressure ratio of approximately 1.9 corresponds with the speed of sound, at which point the flow will choke at the throat and the nozzle. Typical subsonic aircraft have nozzle pressure ratios (NPR) of less than 1.9 at takeoff conditions and thus are unchoked at those conditions. At higher speeds such as at a Mach 0.80 cruise condition, the nozzle pressure ratio rises to approximately 3 and the flow is choked at the throat. At supersonic conditions the nozzle pressure ratio must rise to much higher NPR, shock cells will form outside the nozzle and significant losses in nozzle thrust coefficient will occur, reducing the maximum ideal thrust that could occur.
To achieve high thrust coefficient at high NPR, the nozzle must have some effective divergence, —a convergent-divergent (C-D) such as a De Laval nozzle shown in FIG. 2. Here the minimum throat section 19 is followed by an expansion section to an exit plane of greater area 20 allowing the additional expansion of supersonic flow to apply positive pressure to the diverging walls and allow more complete expansion and maximize thrust.
The graph in FIG. 3 illustrates the performance of simple convergent and divergent nozzles. Ct or gross thrust coefficient is the ratio of thrust realized by the nozzle divided by the thrust obtainable with perfect ideal expansion. The ratio “AR1” represents the ratio of the divergent exit area 20 divided by the throat area 19. As seen, a simple convergent nozzle has high efficiency in the range needed for typical subsonic aircraft, but falls dramatically with the high NPR's required for supersonic flight. Conversely, the C-D nozzle has very poor efficiency at the low NPR's needed for takeoff and any subsonic conditions. In addition, at the low NPR's the simple fixed C-D nozzle will “overexpand” resulting in strong shock cells and very high jet noise.
FIG. 4 illustrates a cross-section incorporating a plug nozzle cross-section. Plug nozzles exhibit the characteristics of a convergent nozzle near and below critical NPR, and at high supercritical NPR the supersonic expansion would provide pressure on the external surface of the plug body and recover the additional thrust otherwise lost in a convergent nozzle. In the FIG. 5 graph below “AR1” represents the internal expansion ratio of area of the annulus between the nozzle cowl exit after subtracting the area of a fixed plug shown in FIG. 4 at the exit plane 16 and the minimum throat area 10 If the peak cross-section of the plug is coincident with the exit plane, AR1=1.0 and there is no internal divergence. “AR2” represents the total expansion ratio including the expansion occurring on the external plug surface aft of the cowl exit plane. It is prescribed as the total cross sectional area at the cowl exit plane 16 of FIG. 4 without the area of the plug subtracted divided by the minimum throat area 10 approximating effective divergence area ratio of an equivalent C-D nozzle.
FIG. 5 illustrates the performance of a fixed plug nozzle such as in FIG. 4 with AR1=1.0 and AR2=1.4.
Supersonic aircraft usually need to change the basic minimum throat area for two reasons. First, at supersonic cruise the incoming airflow temperature rises and increases the power required to drive the fan and compressors without a requisite increase available for temperature limits in the engine. The result is with a fixed nozzle throat area the engine speed and airflow tends to “lapse” to lower values than the inlet is designed for. At supersonic speeds the excess flow approaching the inlet must either be bypassed (around the engine) or spilled ahead of the inlet. Supersonic spillage drag is excessive (beyond the small spillage needed for inlet stability), and a method to avoid it is to “high flow” the engine by increasing the nozzle throat area to induce 100% of nominal air flow through the engine at lower fan pressure ratio. The minimum throat area can be changed in a plug nozzle by appropriate shaping of the surfaces and sliding the plug horizontally into the cowl to increase the throat area. This variability can also be obtained by sliding the external cowl relative to the plug.
The first operational jet fighter ME-262 employed a sliding plug arrangement as part of the engine control, which used a sliding plug that slid into the cowl, to thereby allow “high flow” of the engine by increasing the nozzle throat area, as discussed above.
A more recent impetus for changing the throat area is to reduce external jet noise. Engines designed for efficient supersonic cruise generate very high jet velocities and noise if operated to their maximum airflow and pressure ratio. Reducing noise to acceptable levels requires reducing thrust levels and corresponding jet velocity. With a typical fixed nozzle, reducing thrust also reduces airflow. Since jet noise is dominated by jet velocity more than engine airflow, the minimum jet noise for a given thrust level will occur with the maximum airflow and minimum jet velocity available for a given engine design. This essentially means that for takeoff conditions low noise is also favored by “high flowing” the engine by increasing the airflow to the maximum and minimizing jet velocity to attain the necessary thrust. Increased nozzle exit area causes the fan flow to increase, and the fan pressure to decline, as the fan departs from its optimum “operating line”. The nozzle exit velocity decreases in consequence of the lower fan pressure, resulting in the lower noise exhaust jet.
As shown in FIG. 5, the typical fixed plug nozzle has a minimum thrust coefficient near NPR of 3, corresponding to the high subsonic and transonic speed regimes. This dip in the curve can be improved by applying a small divergence between the throat and the exit plane. The ideal expansion ratio versus NPR is illustrated in FIG. 6.